This invention relates to a system which continuously monitors and analyzes the quality of transmitted radio signals, and more particularly to a system to monitor and analyze the parameters of the signals of an instrument landing system.
The Federal Aviation Administration (FAA) requires that certain parameters of Instrument Landing Systems (ILS) transmitters must be monitored for changes to insure the quality of the radiated signal.
Instrument Landing Systems are ground based transmission systems that provide runway approach information to aircraft. The system consists of two (2) major components, the localizer station, which provides runway centerline information, and the glideslope station, which provides glidepath information.
The ILS signal is modulated with two (2) audio tones, a 90 Hz tone and a 150 Hz tone, that are equal level. Complex antenna radiation patterns and separately radiated sideband components modify the modulation of the tones so that they are of different levels at different points of the pattern. This difference in the levels, when processed by the aircraft receiver, creates an approach corridor. This corridor is oriented along extended runway centerline, has a specific angle, and horizontal and vertical width. For localizers, if an aircraft is on centerline, the 90 Hz and 150 Hz will have equal levels and the receiver crosspointer will be centered. As it flies to the left of centerline, the level of the 90 Hz will increase and the 150 Hz will decrease. This excess of 90 Hz will generate a fly right indication in the receiver. The opposite happens on the right of centerline. The glideslope operates in a similar manner, with an excess of 90 Hz above glidepath, and an excess of 150 Hz below.
Both the localizer and glideslope stations have complex monitoring systems that continuously check the parameters of the ILS signal. The monitors look for changes in RF level, total percent modulation, and the difference of the modulation of the two (2) tones at selected points of the radiated pattern. If there is a change of any parameter that exceeds tolerances established by the Federal Aviation Administration, the monitors automatically initiate actions to remove the defective equipment from service.
The approach information transmitted by the ILS is in the difference of the modulation of the two (2) tones. This difference is called the difference of the depth of modulation, or DDM, and is the modulation factor of one (1) tone subtracted from the modulation of the other tone. Modulation factor is the ratio of the audio peak voltage to the RF level, and is described by the formula Va/Vc, where Va is the peak amplitude of the audio, and Vc is the amplitude of the unmodulated carrier. Percent modulation is the modulation factor multiplied by 100.
The localizer transmitted signal has the audio modulated at 20 percent per tone. This means that the 90 Hz is at a modulation factor of 0.2000 and the 150 Hz is at a modulation factor of 0.200. This condition of equal modulation is called modulation balance. The DDM in this condition is 0.200 minus 0.200, or 0.000 DDM. In the radiation pattern, this is the condition on runway centerline. The total percent modulation is the sum of the modulation factors times 100, or (0.200+0.200).times.100=40%.
At the width points, as established by FAA, the modulation factor of one (1) tone is 0.1225, and the other is 0.2775. The DDM is 0.2775-0.1225=0.155 DDM, and the total modulation is (0.2775+0.1225).times.100, or 40%. Note that as the DDM changes the percent modulation doesn't change.
The composite 90 Hz and 150 Hz is a complex waveform, that is described by the formula: Sin 3X+Sin 5X. This waveform, as shown in FIG. 1, repeats itself 30 times a second, and has a phase relationship such that the peak of the 90 Hz does not coincide with the peak of the 150 Hz. Because of the phase relationship, if the formula Va/Vc is applied to this composite signal at 20% per tone, the percent modulation would appear to be 37% instead of 40%.
The monitors check the transmitted signal through a linear detector. The detector changes the RF to a DC voltage. The detected signal is the composite 90 Hz and the 150 Hz on a DC level, as shown in FIG. 1.
The present monitoring methods are analog in nature. They check the parameters by separating the individual components of the signal, then electronically performing the modulation formula to get the required information.
The detected signal usually goes through several stages of amplification. From the amplifiers, it is sent through a filter that separates the audio from the DC level (the RF level, or Vc). The audio is sent through another filter that separates the 90 Hz from the 150 Hz. The two (2) tones are detected and filtered to produce DC voltages that represent the peaks of the audio (Va). Having both components of the basic formula, they are applied to electronic math circuits to get percent modulation and DDM. RF level is an arbitrary value that is monitored for change.